The invention relates to a cathode comprising a support of an alloy comprising mainly nickel and coated with a layer of electron emissive material comprising alkaline earth metal oxides and barium.
Such cathodes are generally known and are described, for example in "Advances in Electronics and Electron Physics" 25, 211-275 (1968). The emission of such cathodes is based on the release of barium from barium oxide. In addition to the barium oxide, the electron-emissive material usually comprises strontium oxide and sometimes calcium oxide.
The actual emission is mainly ensured by small regions (so-called "sites") having the lowest effective work function for electrons which are spread over the electron-emissive material. In practice, sites having a slightly higher work function will hardly contribute to the electron current generated by the cathode.
For a high effective electron emission it is therefore favourable to increase as much as possible the number of sites having a minimum possible work function in the total distribution of sites.
The addition of samarium oxide and thulium oxide and oxides of some other rare earth metals is proposed in European Patent Application EP 0,210,805 for the purpose of life-time improvements, with scandium oxide or yttrium oxide being preferred.
However, the additions mentioned in this Application are found to yield a very small or no zero-hour improvement and this may even be at the expense of a certain deterioration in the initial emission, notably with scandium oxide (see also, for example, FIG. 3 in EP 0,204,477).